Synchronous electric machine with superconductive field windings

ABSTRACT

A synchronous electrical machine including a fixed superconducting field winding to provide D.C. magnetic field when a D.C. current circulates through the winding. A fixed normally conducting armature is provided with a plurality of spaced-apart disks. Each disk is made from insulating material and is provided with flat windings for circulation of A.C. currents. A plurality of rotor disks is axially interleaved with the disks of the armature. Each rotor disk produces A.C. electromagnetic forces in the flat windings upon rotation of the rotor.

The invention relates to synchronous electric machines (alternator ormotor) of the type comprising a fixed superconductive inductor throughwhich flows a DC current for creating a continuous magnetic field withaxial symmetry about the axis of rotation of the rotor and a woundarmature with normal conductivity through which flow multi-phasealternating currents during operation, as well as a rotor comprisingferromagnetic masses rotating in the field of the field windings so asto create alternating electromotive forces in the fixed windings, withnormal conductivity.

Rotary machines of the above type are already known which have theadvantage, over rotary superconductor machines with rotary fieldwindings, of doing away with the need for rotary seals in the circuit ofthe cryogenic fluid for cooling the inductor. None of these knownmachines is able to form a high powered machine. The article by J. Huretet al. in "Bulletin de la Societe Francaise des Electriciens", vol 5,October 1964, No 58, pp 654 and 655 describes an alternator which is thetransposition to the cryogenic field of conventional claw alternators inthe automobile field: this is a machine whose rotor comprises a core inthe form of a double cross, the two crosses surrounding the stator. Thedynamo described in "PHILIPS Research Reports", vol 23, No 3, June 1968,pp 249-269 is of the homopolar type not comprising any armature withnormal conductivity. The document FR-A-2 280 998 describes a rotaryelectric machine whose field windings and armature are both made from asuperconductive material.

The present invention aims at providing a synchronous electric machinewith superconductor answering better the requirements of practice thanthose previously known, especially in that it readily allows industrialpowers to be reached.

To this end, the invention proposes more especially a machine in whichthe rotor is situated radially inside said wound armature and comprisesseveral evenly spaced apart disks in the axial direction each of whichcomprises several ferromagnetic material masses spaced angularly evenlyapart, the masses of all the disks being aligned.

Because of the use of a superconductive field winding, a very highpolarizing field may be provided, permanently saturating theferromagnetic material, which allows a low inductance value for thearmature to be obtained, whatever the value of the armature currents. Nosliding contact is required. The rotor may be formed by a massive piecewithstanding well the centrifugal forces.

As a general rule, the field windings will be formed by a windingsimilar to that of a solenoid, creating a magnetic field with axialsymmetry about the axis of rotation of the rotor. This latter will haveto be centered accurately in the field windings for any deviation givesrise to electromagnetic forces which tend to increase it further duringoperation.

A screen made from an electricity conducting material will generally beprovided between the wound armature and the field windings so as toprotect this latter against the induction variations during operation ofthe machine.

The rotor may then be formed by a series of disks provided with evenlyspaced ferromagnetic masses and supported by a common shaft forinputting or outputting mechanical power depending on whether themachine forms an alternator or a motor. The wound armature will havewindings made from a material with normal conductivity, may then bedisposed on fixed insulating disks, alternating with the disks of therotor.

The invention will be better understood from reading the followingdescription of a machine which forms a particular embodiment of theinvention, given by way of example. The description refers to theaccompanying drawings in which:

FIG. 1 is a simplified diagram of a rotary machine shown in sectionthrough a vertical plane;

FIG. 2 is a left-hand view of the machine, with the rotor removed;

FIGS. 3 and 4 are schematical views showing, respectively in aright-hand view and in section, one of the disks of the armaturewindings;

FIG. 5 is a simplified diagram showing one possible rotor construction,the disks which it supports being shown in section;

FIG. 6 is a left-hand view of the rotor of FIG. 5;

FIG. 7, similar to FIG. 3, shows an armature winding disk forming avariation:

FIG. 8 is a schematical perspective view of the disk of FIG. 7;

FIG. 9, similar to FIG. 6, shows a rotor disk usable with the fixed diskof FIG. 7.

The rotary machine shown in FIGS. 1 and 2, which may be used asalternator or as motor, comprises a superconductive field windings 10through which there flows permanently a DC current, fixed multiphasenormally conducting armature windings 12, which are seats of alternatingelectromotive forces during operation, and a rotor 11 provided withferromagnetic masses which, under the effect of the field produced bythe field windings, acquire magnetization.

These different components will now be described successively as well asthe members which are directly associated therewith.

The superconductive field windings 10 shown in FIGS. 1 and 2 is formedby a cylindrical winding having an axis 13 along the axis of rotation ofrotor 11. This winding will be formed from a conventionalsuperconductive material, for example niobium-titanium, wound on aninsulating mandrel in accordance with anyone of the different well-knowntechnologies. This winding is placed in a liquid helium cryostat 14,also of conventional construction, having a liquid helium supply well15. This cryostat 14, also constructed according to conventionaltechniques used for example for superconductive electromagnets forguiding particle beams, has a cylindrical internal wall 16 defining achamber around axis 13 which forms an uncooled space.

To protect the superconductive winding 10 and the cooled metal parts ofthe cryostat against the variable magnetic fields produced duringoperation of the machine, the internal wall 16 is covered on the insideby a fixed metal screen 17 made from a good electricity conductingmaterial. The thickness of screen 17 must be sufficient for the eddycurrents which appear to compensate for the alternating fields producedby the armature windings 12 and by rotation of rotor 11. As a generalrule, it is sufficient to give screen 17 a thickness twice the skineffect depth for the frequencies contemplated. For an alternatoroperating at 50 Hz, a copper screen having a thickness of about 1 cm isgenerally sufficient. Since the screen is likely to be subjected to highforces, it must be firmly fixed to the frame of the machine.

There may be further provided, around field windings 10 an additionalscreen for protecting the environment from the magnetic fields which itproduces; but in practice, this field is not very troublesome, becauseit is continuous and this outer screen may be omitted since it wouldhave to be very heavy to be efficient and would require accuratecentering.

Rotor 11 may be solid or, on the contrary, in the form of a shaft 18 onwhich are secured one or more disks 19, three in number in theembodiment shown in FIGS. 1 and 5. Disks 19 are identical and are spacedapart at equal intervals. Sector shaped masses 20 are formed or added todisks 19 at regular intervals. These masses, twelve in number in theembodiment shown in FIG. 6, must be made from a ferromagnetic material,as are also disks 19 as a rule. This material must be chosen with thehighest possible saturation magentization,. In practice, soft iron or aniron-cobalt alloy will generally be used. The field provided by thefield windings 10 will have to be such that the material always presentsits saturation magnetization (2.1 Teslas, in the case of soft iron).Thus, the magnetization is not disturbed, at least in the first order,by the armature reaction field and this latter will have no unfavorableinfluence on the electromotive force.

The armature windings 12 alternate with and overlap rotor disks 19 suchthat the internal windings 12 are interleaved or are interfitted betweenthe disks 19 of the rotor. The windings 12 properly speaking may beformed on flat insulating supports in the form of disks 21 made forexample from glass fiber reinforced resin. These disks 21 are fixed tothe structure of the machine, for example by forming a stack of thesedisks 21 and distance pieces or spacers 22 (FIG. 1) which are clampedbetween end flanges (not shown). The insulating disks 21 are provided oneach face with radial grooves in which are disposed conductors 23 (FIG.3) for forming the multiphase multipole windings constituting thearmature windings 12. These conductors of the armature windings aredisposed and connected in accordance with the usual rules for formingwindings for synchronous machines with flat geometry and are connectedto the power terminals of the machine (not shown).

Masses 20 (FIG. 6) and the armature windings will have shapes adapted toeach other so as to provide electromotive forces whose law of variationin time complies with requirements. When in particular the machine is tobe coupled to the grid, these electromotive forces must be approximatelysinusoidal. This result may typically be reached by placing theconductors of the armature windings in notches which give them the shapeof approximately trapezoidal sectors, whereas masses 20 have a roundedshape, so that the winding surface covered by one of the masses isapproximately sine shaped. But numerous shapes may be used differentfrom those shown by way of examples.

The space occupied in the axial direction by the disks 19 of rotor 11and by the armature windings 12 will be the result of a compromise: ifthe thickness of the fixed disk 21 is reduced, the electromotive forceis increased, but, on the other hand, the armature currents and so thepower of the machine are reduced. In practice, the axial dimension ofthe disks of rotor 11 and the spacing apart of the masses 20 which formit will be of the same order of magnitude. For each flat disk 21provided with its windings, an axial length may be accepted of about aquarter of the axial space occupied by the disks and the masses of rotor11.

To dissipate the heat released by Joule effect in the armature windings12, it will frequently be necessary to provide, in these windings, acooling circuit which is not shown, for it may be of a conventionaltype.

Whatever the embodiment chosen, it allows all the advantages ofsuperconductive field windings machines to be kept, i.e. particularly amuch smaller synchronous inductance than that of the large machine ofconventional type, which reduces considerably the absorbed reactivepower and increases the maximum performances, because one of the usuallimitations of the maximum armature current is avoided (reducedstability because of the increase of the internal angle during permanentoperation).

With respect to these machines, the machine of the invention presentsthe above-mentioned advantages. With respect to conventional synchronousmachines whose windings are made from a material having normalconductivity, the invention presents the advantage of providing amachine whose ferromagnetic masses permanently completely saturated, sothat their differential permeability is close to the permeability of avacuum.

In the modified embodiment shown in FIGS. 7, 8 and 9, the disks of rotor11 are provided with masses 20 in the form of trapezoidal sectors. Thearmature windings 12 are formed from conductors, preferably flat bars,housed in grooves 24 in the form of involutes of a circle hollowed outin the lateral faces of the fixed insulating disks. The opposite facesof the same fixed disk are provided with grooves 24 orientated in thereverse direction which allows an undulating winding to be formed foreach of the phases by connecting the flat conductors to the external orinternal periphery of the disks 21.

This armature may for example be cooled by forced longitudinal flow of acooling gas through holes (not shown) formed in the disks at theposition of grooves 24. These holes have a diameter greater than thewidth of the grooves which allows the conductors to be cooled by lateralcontact of the gas with the flat bars.

The invention is susceptible of numerous other variations, relating notonly to the construction of the masses and of the armature windings butalso to the technological construction of the components. The armaturewindings may be disposed with respect to the superconductive inductor ina different way from the one shown. Screen 17 made from a conductingmaterial (copper or aluminium alloy) may have a varying thickness, so asto provide a maximum attenuation where it is required, for example itmay be thicker in its endmost parts. It goes without saying that suchvariations, as well more generally as others remaining within the scopeof equivalences, must be considered as covered by the present patent.

I claim:
 1. A synchronous rotating electrical machine comprising, incombination:a fixed stationary superconductive field winding adapted toprovide a high intensity D.C. magnetic field having flux linessubstantially parallel to an axis and a rotational symmetry about saidaxis in a region around said axis when a D.C. current circulates throughsaid field winding; a fixed normally conducting armature having aplurality of disks spaced along said axis, each disk made from anelectric insulating material and having flat windings located in saidregion for circulation of multiphase A.C. currents; and a rotor mountedfor rotation about said axis, having a plurality of rotor disks axiallyinterleaved with the disks of said wound armature, each said rotor diskcarrying a plurality of identical ferromagnetic bodies evenlydistributed angularly about said axis and located in said region forcreating A.C. electromagnetic forces in said flat windings upon rotationof said rotor.
 2. The machine according to claim 1, wherein said fieldwinding is adapted to provide a magnetic field sufficient to maintainsaid ferromagnetic bodies permanently beyond saturation.
 3. The machineaccording to claim 1, further comprising a fixed cylindrical conductingscreen coaxial with the rotor.
 4. A machine according to claim 1,wherein each of said armature disks is comprised of a flat support ofglass fiber reinforced resin formed with radial grooves for receivingconductors of said flat windings.
 5. A machine according to claim 4,wherein said ferromagnetic bodies have a rounded shape and the armaturewindings are approximately trapezoidal sectors in shape, wherebyelectromagnetic forces having sine-shaped variation are obtained.